RC snubber circuit in the switching power supply design
Time:2013-03-21
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Switching power supply with a transformer in the topology, the switch is turned off, the voltage and current caused by the loss of overlap is a major part of the switching power losses, while, due to circuit stray inductance and stray capacitance, the power switch off off, the circuit will also appear over-voltage and oscillation. If the peak voltage is too high, it will damage the switch. Meanwhile, the existence of oscillation will increase output ripple. To reduce the turn-off losses and peak voltage, you need to switch both ends of the parallel snubber circuit to improve the circuit performance.
The main function of the buffer circuit are: First, to reduce turn-on or turn-off losses; second, lower voltage or current spikes; third is to reduce the dV / dt or dI / dt. MOSFET, the current decline as fast, so it's a very small turn-off losses. While still using the MOSFET turn-off control buffer circuit, but its role is not to reduce the turn-off losses, but reduce the transformer leakage inductance voltage spike. This paper tubes for the MOSFET turn-off snubber circuit to be discussed.
1 RC snubber circuit design
RC snubber circuit design must be familiar with the main circuit topology used in the situation. Figure l shows the composition of the RC snubber forward converter. In the figure, when Q is off, the collector voltage begins to rise to 2Vdc, and capacitor C limits the collector voltage rise rate, and reduced the rise in voltage and decrease current overlap, thus reducing the Q-switch loss. But before the next switch off, C must have full voltage 2Vdc done and the discharge path for the C, Q, R.
Suppose switch with no buffer circuit shown in Figure 1, the reset forward converter primary coil winding and the same number of turns. Thus, when Q off instantly, stored in the magnetizing inductance and leakage inductance of the energy release, the reverse polarity voltage across the primary winding, the switch forward converter collector voltage quickly rose to 2Vdc. Meanwhile, the excitation current flows through diode D reset winding, and finally reduced to zero, then the voltage across Q down to Vdc. Figure 2 shows the switch collector current and voltage waveforms. Visible, switch circuit without the buffer, the Q is off, the leakage inductance voltage spikes across the great, the resulting turn-off losses are large, severe burn switch is likely, therefore, must be to switch tube with buffer circuit.
When the switch with a snubber circuit, its collector voltage and current waveforms shown in Figure 3 (in forward converter as an example).
In Figure 1, when Q started off, its current began to decrease, while the transformer leakage inductance will prevent the current decreases. Part of the current will continue to be off the switch, and partly by the RC snubber circuit and charging capacitor C, resistor R of size and charge current.
Ic part of the flow into the capacitor C, can slow down the collector voltage to rise. By selecting a large enough C, can reduce the increase in collector voltage and current overlap decreased to significantly reduce the switch turn-off losses, but also can inhibit the collector leakage inductance voltage spike.
Figure 3 in the AC stage switching stage is turned off, CD on-stage for the switching control. Before the switch is turned off, the voltage across capacitor C is zero. In the turn-off time (B time), C collector voltage will slow the rate of increase, but also be charged to the 2Vdc (ignore the moment in the leakage inductance voltage spike case).
The size of capacitor C not only affects the collector voltage rise speed, but also determines the resistance R on the energy loss. Q off in an instant, C the voltage 2Vdc, its stored energy to 0.5C (2Vdc) 2 joules. If all the energy consumed in R, then weekly during the energy consumed in R, as follows:
Collector voltage is increased restrictions, C should be the bigger the better; from the starting efficiency of the system, C the greater the loss the greater the efficiency is lower. Therefore, we must select the appropriate C, so that it can achieve some increase in collector voltage slow pace of the role, it does not cause loss of the system is too large Ershi low efficiency.
In Figure 3, due to start off the next time (D time) must ensure that there is no voltage across C, so, in the B to D time between the moment a certain period of time, C must discharge. In fact, the capacitor C in the CD this time, but also by the Q through resistor R and R constitute the discharge circuit to discharge. So, the choice of a large enough, after C, R C should be made at the minimum on-time ton load within the discharge to the desired charge of 5% or less, so there are:
Type (1) that R, and C the energy loss is proportional and, therefore, must choose the appropriate C, so how to choose C RC snubber circuit design has become the key, here a more practical choice of capacitor C, Methods.
In fact, when Q started off, the assumption that the first half of the peak current Ip flowing through C, the other half is still flowing through the gradual shutdown of the Q collector, and assuming that the transformer leakage inductance to maintain the total current remains at Ip. Then, by selecting the appropriate capacitor C, in order to switch within the collector voltage rise time tf to 2Vdc (where tf is the collector current dropped to zero from the initial value of the time, you can switch the data from the manual check), have:
Therefore, from (1) and type (3) be able to calculate the size of capacitor C. After determining the C, while the minimum on-time is known, so that by equation (2) The resistance R can be the size.
2 RC buffer with the main forward converter circuit design
2.1 Circuit Design
Figure 4 shows an RC snubber circuit with forward converter primary circuit. The main circuit parameters: Np = Nr = 43 turns. Ns = 32 turns, switching frequency f = 70 kHz, input voltage range of DC 48 ~ 96 V, output DC 12 V and DC 0.5 A.
Q switch for the MOSFET, model IRF830, its tf typically 30 ns.
Dl, D2, D3 for the fast recovery diodes, the tf is small (typically tf = 30 ns).
The design of the output power P0 = V0I0 = 6 W, assuming 80% efficiency of converter, each way by the loss of RC snubber circuit power accounts for 1% of output power. Here take Vdc = 48 V.
2.2 Experimental Analysis
The following are two situations of the design of experiments, one primary winding with buffer, sub-free buffer; second primary non-buffer, a secondary buffer.
(1) the primary winding has buffer, no buffer sub-
The experiment measures the switch at both ends of the drain-source voltage of Q, were divided in two situations:
The first is RS1 = 1.5 kΩ, CS1 variable input DC voltage Vdc to 48 V.
The experimental results are as follows: in the RS1 unchanged, CSl larger, although the Q-switch leakage inductance voltage spikes did not decrease significantly, but its drain-source voltage becomes flat, and this shows the RC in the primary switch buffer circuit, CSl should choose a smaller value.
The second situation is CSl = 33 pF, RS1 variable input DC voltage Vdc to 48 V. The result: when CS1 is constant, RS1 larger switch Q, the greater the leakage inductance voltage spike (an increase of small).
Visible, RC snubber circuit, the parameter R the size of the lower leakage inductance spike has a great influence. In selecting a suitable C, also satisfy (2),, R should be chosen small value.
(2) secondary windings are buffered, unbuffered primary
In this experiment, D2, D3 of the cathode as a public side to measure the fast recovery diode-side pressure, the result is that when R is constant, C the greater the diode leakage inductance spike at both ends of the smaller. At the same time in theory, if C is infinite, the voltage across the diode in a sense no tainted peak. In practice, only allow the voltage across the diode's leakage inductance voltage spike at its end within 30% of the peak pressure to meet the requirements, and it also will not cost too much.
2.3 The design parameters
Through experimental analysis shows that, in the sub-fast recovery diode RC snubber circuit, when selected the appropriate size of capacitor C, in meeting the type (2) of the case, the resistor R should be chosen as small as possible.
After the final actual debugging, the choice of RC snubber circuit design parameters are:
Primary: RS1 = 200, CSl = 100 pF
Secondary: RS2 = RS3 = 5l, CS2 = CS3 = 1000 pF
The design of the primary switch of the RC snubber circuit in the C values although the election was slightly larger than the calculated value, but the loss is not great, so is acceptable. Relatively elementary, the secondary fast recovery diode RC snubber circuit in the C value calculated on the election much larger than the system loss will inevitably increase. However, fast recovery diodes in parallel across 的 RC buffer circuit mainly yes output to improve system performance, so Xuanze relatively large in the C Zhisui Ran make the overall system efficiency decreased, but a sense of diode leakage at both ends of the peak on the Jian Xiao a lot and output voltage ripple can also meet specified requirements.
3 Conclusion
According to the formula given above, can be very good and very easy to choose the right RC snubber circuit. However, in engineering applications, system design should be based on performance, through the actual commissioning can be really appropriate parameters. Sometimes, in order to achieve the system performance, the expense of some efficiency is necessary. In short, in the design of RC snubber circuit parameters must be taken into account the system performance and efficiency, ultimately choose the right RC parameters.
